build-services

Legacy feature (has `data/state/` or `services/main-service/`)? Extend it in place per the legacy rules; don't migrate unless asked. Skip if the feature talks to nothing outside itself. Create one `services/<name>-service/` namespace per service: the interface (an async data service, validated with `AsyncDataService.IsValid`), `create` (the implementation, or a factory that throws when the app must inject it), `createFake`, and `public.ts`. Add `services/services.ts` with the `Services` map when spec actions inject services. Depends only on `data/`. Gate: per-service unit tests. The how is in the auto-loading `features/services/index.md` rule.